A few cold drops falling through a cloud could create a downpour

A lab-made cloud has thrown light on a mysterious rain-creating phenomenon first witnessed decades ago. Atmospheric scientists haven’t fully understood the mechanism by which a single cold droplet or ice shard can trigger abrupt rain or snowfall – as it did in an experiment in the 1940s when scientists dropped chunks of dry ice from an aeroplane through cumulus clouds. Now, Prasanth Prabhakaran at the Max Planck Institute for Dynamics and Self-Organization in Germany and his colleagues have used an artificial cloud in a box to simulate the phenomenon, finding that a falling drop of cold water can trigger the growth of minuscule droplets along its path. Advertisement In real clouds, water drops generally grow by sticking to aerosol particles. But Prabhakaran and his colleagues created artificial clouds in which liquid droplets at high pressure can grow on their own without the need for any aerosols, whose addition would also have made the cloud difficult to model. To further simplify the experiment, the researchers used two chemical stand-ins. Helium took the part of nitrogen and other gases, while sulphur hexafluoride played the role of atmospheric water, because it can be both a liquid and a vapour, and it forms droplets at lower temperatures and pressures than water. The cloud was created in a special box with a heated bottom and a top that was 4°C cooler. Some sulphur hexafluoride formed a pool at the bottom of the box, while some evaporated into the helium gas above. Drops of sulphur hexafluoride condensed on the ceiling of the box and dropped back down through the gaseous cloud. Because the drops had formed on the cooled top of the box, they were colder than the surrounding gas. As a drop fell, it cooled the area around it. That caused lots of “microdroplets” to form in its wake, just as condensation forms on a cold can when it comes into contact with warmer air. Prabhakaran and his colleagues say this mechanism could help explain some abrupt rainstorms. Depending on atmospheric conditions, microdroplets produced this way in real clouds could merge until they are big enough to fall as a raindrop, or be swept into an updraft and then create microdroplets of their own. But because clouds have turbulent flows of air and varying temperatures, and because they contain particles of different sizes, it is tough to tell how realistic this model is. “I see little relevance of this study to real-world clouds,” says Daniel Rosenfeld at the Hebrew University of Jerusalem in Israel. Because the artificial cloud is so small, the processes that happen there may not scale up to the size of actual clouds in Earth’s atmosphere, he says. Jonathan Crosier at the University of Manchester, UK, says if this is what happens in real clouds, it may be a rare phenomenon. “I’m not convinced it’s an important process in terms of understanding how 99.9 per cent of the droplets in a cloud are formed,” he says. “However, to really know, detailed simulations are needed in a very wide range of scenarios.” Read more: Laser creates clouds over Germany More on these topics: atmosphere environment weatherUtne Altwire: science